Non-contact SQUID-NDT method using a ferrite core for carbon-fibre composites

Carbon-fibre composites (CFCs), such as carbon-fibre-reinforced plastic (CFRP), are promising composite materials for aerospace structures. However, there is no reliable non-contact NDT method for the detection of deep-lying cracks in thick CFCs at the present time. In this study a non-contact eddy-current-based SQUID-NDT method for thick CFCs was developed. Because CFC is conductively low (electrically), and the target CFC is thick, an induction coil with a U-shaped ferrite core was employed to generate a strong induction field while supplying a low frequency current to the coil. This method was applied to 20 mm thick CFRP specimens with hidden slots at various depths. All signal responses due to the slots located at 5 mm up to 17.5 mm in depth were successfully detected while supplying 150 mA at 300 Hz. The peak amplitude of the response obtained by the method was the same as, or larger than, that of previous results on the same specimens by the current injection method. It shows that the developed method can efficiently induce a large eddy current in the conductively low specimen. It is concluded that this method has the potential to be applicable to the non-contact NDT on very thick CFCs.